Local area networks (LANs) have been used to facilitate communications between end users connected to the same physical network. Individual LANs may be bridged to allow end users to communicate with other end users connected to different networks using layer 2 switches. Bridged LANs may be further interconnected with other bridged LANs using routers to form even larger communications networks. However, bridging and routing increase the processing load on machines that interconnect the various physical networks.
VLANs have been developed to address various deficiencies in bridged and routed networks and to allow LANs to be bridged in virtually any desired manner, independently of physical topology of the networks. VLANs can contain network traffic to a predefined set of ports in a layer 2 switch, thus eliminating the unnecessary use of bandwidth by broadcast and multicast frames. For example, layer 2 traffic that arrives at a switch is preferably only flooded onto output ports associated with the same VLAN. Without VLANs, layer 2 traffic for which no layer 2 forwarding database entry exists is flooded onto all output ports. Thus, using VLANs allows traffic to be contained within each VLAN for flooding purposes. The VLAN for a received frame is identified by a VLAN tag or the assigned VLAN tag for the receiving port. In most common VLAN implementations, the VLAN tag is a four-byte field between the link layer header and the network layer header of a frame. The VLAN tag contains a VLAN identifier and an associated user priority field. The term VLAN tag, as used herein, is intended to refer to the VLAN identifier portion of the VLAN tag, even though it is understood that a VLAN tag may include additional information.
Using current VLAN implementations, a layer 2 switch is not permitted to change the VLAN tag in an incoming frame to another VLAN tag in an outgoing frame. As a result, in order to interconnect VLANs that use different VLAN tags, it is necessary to provide a mechanism for mapping or coordinating VLAN tags used by the different networks. One potential solution to this problem is to stack VLAN tags in each frame. That is, when a frame arrives at a switch from a first VLAN and is destined for a second VLAN, the switch may add a second VLAN tag to the frame in addition to the first VLAN tag. The frame will be switched in the second VLAN using the second VLAN tag. When the frame leaves the second VLAN, the second VLAN tag is removed.
One problem with using VLAN tag stacking is that tag stacking only provides a mechanism for switching frames in the interconnecting network. When frames leave the interconnecting network, the same VLAN tags that were in the frames when they entered the interconnecting network must be used. In other words, there is no ability to expand the broadcast or flooding domain associated with the original VLANs. Another problem with tag stacking is that it increases the complexity in decoding frames in the interconnecting network. Accordingly, there exists a need for improved methods and systems for associating and translating VLAN tags in layer 2 frames.